1. Field of the Invention
This invention relates to fuel rods for fuel assemblies used in light water nuclear reactors.
2. Description of the Prior Art
Fuel assemblies for light water nuclear reactors comprise an array of cylindrical rods termed fuel rods. Each fuel rod contains a stack of pellets made of a fissile material such as uranium, especially uranium 235.
The reaction which produces energy in a nuclear reactor is the neutron-induced fission of uranium atoms. The fission reaction rate has a spatial distribution which is affected by the neutron flux distribution. Because the distribution of fission-inducing neutrons is approximately cosinosoidal along the axis of the fuel rods, the depletion of fissile fuel, or burnup, distribution along the length of a fuel rod is non-uniform, with the ends of the rod receiving less burnup than the center. This is inefficient utilization of the uranium 235 in the fuel rod ends.
One well-known improvement is to add pellets comprising naturally enriched uranium (0.711 weight percent U-235 in U-238) or depleted uranium (containing less than 0.711 wt. % U-235) at the rod ends. These pellets are called axial blanket pellets and function to generate plutonium by the absorption of neutrons by the uranium 238. Because plutonium is fissile, the blanket pellets improve the burnup distribution somewhat.
It is desired to provide an improved fuel rod design which accomplishes an improved axial burnup distribution.